1. Field of the Invention
This invention relates to hydrophilic porous structures, and processes for their production.
2. Description of the Prior Art
Fluorocarbon resins, in spite of their excellent heat resistance and chemical resistance, are difficult to apply to systems involving water and aqueous salt solutions because they are hydrophobic. In an attempt to render a thin film of a fluorocarbon resin hydrophilic, Japanese Patent Publication No. 20742/61 and U.S. Pat. No. 3,666,693 disclose methods which involve grafting acrylic acid, 4-vinylpyridine, N-vinylpyrrolidone, etc. to the fluorocarbon resin using an ionizing radiation-induced or catalyzed polymerization reaction. According to these methods, however, graft copolymerization proceeds on the surface layer of the resin, but does not easily proceed in the interior of the resin. Hence, these methods cannot be used to achieve uniform hydrophilicity. On the other hand, U.S. Pat. Nos. 3,390,067 and 3,632,387 disclose that in order to extend graft polymerization into the interior of porous polytetrafluoroethylene, it is impregnated with a surface active agent and etched with sodium-naphthalene, etc., after which a polymerizable monomer is graft-copolymerized to the treated polytetrafluoroethylene thereby to render the polymer hydrophilic. Uniform hydrophilicity can be achieved using these methods when the polytetrafluoroethylene has a large pore size as in the case of a felt. However, with a smaller pore size, the etching treatment becomes non-uniform and results in a non-uniform hydrophilicity.
Hydrophilic polymeric membranes are used for separating substances in water-containing systems as membrane filters, dialysis membranes, ultrafiltration membranes, reverse osmosis membranes, etc. For these applications, the polymeric membranes need to have high percent rejection for solute, and superior mechanical strength when wet. Typically, cellulose ester membranes are considered to have superior permeability and mechanical characteristics, but for practical application, these properties should be improved further. The outstanding defects of cellulose ester membranes are their susceptibility to hydrolysis at excessively high acidity or alkalinity, and the decrease in their heat-resistant temperatures with smaller pore sizes.
A polyvinyl alcohol membrane has excellent permeability to water, but a very low mechanical strength. Generally, the permeability of a membrane tends to increase in proportion to the water absorption thereof, whereas the mechanical strength of a membrane tends to decrease with increasing water absorption. It is considered to be extremely difficult therefore to have both characteristics satisfied at the same time. On heat-treatment, a polyvinyl alcohol membrane becomes a water-insoluble membrane having superior permeation characteristics, but the mechanical strength of a polyvinyl alcohol membrane in the wet state is quite weak for practical application. The mechanical strength of a polyvinyl alcohol membrane could be increased by acetalization, etc., but on increasing the mechanical strength thereof, the water absorption of the membrane decreases and the permeability of the membrane is markedly reduced.